Blue shirt guy is holding a can of spray paint. You're supposed to think they're going to harass the homeless guy with it, but they bought him food instead.
These particular characters have deliberately been given stereotypical mannerisms that lead the reader to think that they are the kind of people who would act in such a way. The joke then ends up being on the reader because it turns out that they could not have read these characters and their intentions any more wrong, and illustrates the folly of stereotypes.
If your culture does not have this particular stereotype then I can see how this was lost on you.